Gore studied the evolution of a gene in regards to resistance to the antibiotic cefotaxime (Source: Janice Haney Carr)

Study shows how often and under which conditions reversal of evolutionary adaptations occur in bacteria

Jeff
Gore, study leader and assistant professor of physics at MIT, and a team of
researchers, have worked to figure out what the likelihood of reversing an
evolutionary adaptation would be in bacteria in certain environments.

Charles
Darwin, an English naturalist, proposed his theory of evolution in
1859 in the book, "Origin of Species," which theorizes that all
species of life have descended over time from common ancestry, resulting from
natural selection.

Darwin's
ideas raised a lot of questions, such as whether evolutionary
adaptations can be undone. In 2003, a study showed that some insects
have gained the ability to fly through the use of their wings, then lost it,
then regained it again over millions of years. But then a study conducted a few
years later contradicted this finding through the discovery of a certain
protein, which aids in the control of stress responses and cannot evolve to its
original form.

In
response to these contradictory findings, Gore set out to find whether
evolutionary reversal was possible, and if so, under what circumstances and
what fraction of the time.

Gore
built his research upon a previous study conducted by Harvard University
scientists, which determined that five mutations that are vital for bacteria to
gain resistance to an antibiotic called cefotaxime. With all five mutations,
bacteria are most resistant. With less than five or none, bacteria become more
susceptible. But for those with no mutations, bacteria can gain resistance by
obtaining each of the five mutations.

To obtain
these mutations, evolution must "proceed along a given path if each
mutation along the way offers a survival advantage." These paths are
studied by scientists through fitness landscapes, which are diagrams of
possible genetic states for genes as well as their relative fitness in a
certain environment. For bacteria with no mutations at all, there are 120
possible paths, and findings showed that only 18 ever obtain all five
mutations.

To do
this, Gore first looked at the genetic states. If genetic states differ by only
one mutation, they are always reversible if one state is more fit in a certain
environment and the other state is more fit in another environment. With this
knowledge, the MIT team used computational models and a series of experiments
to figure out how often and under what conditions two states' rate of reversal
decreased as the number of mutations between them increase.

According
to Gore, complex adaptations can be reversed, but they're just more difficult
to reverse. In fact, the study showed that a very small percentage of these
evolutionary adaptations can be reversed if they have four or more
mutations.

"This
is the first case where anyone's been able to say anything about how
reversibility behaves as a function of distance," said Gore. "What we
see in our system is that once the system gets four mutations, it's unable to
get back to where it started."

Gore was
able to show the reversibility between "every possibly intermediate
state" in the researchers' fitness landscape, which not only shows how
likely it is for an evolutionary adaptation to be reversed in certain
environments, but also explains why unneeded organs like the appendix do not
disappear.

"You
can only ever really think about evolution reversing itself if there is a cost
associated with the adaptation," said Gore. "For example, with the
appendix, it may just be that the cost is very small, in which case there's no
selective pressure to get rid of it."